FIG. 9 is an explanatory view showing an example of utilization of an ATM exchanger in the conventional technology, and in this figure, designated at the reference numeral 27 is an ATM exchanger, at 28 is a private branch exchanger (described as PBX hereinafter), at 29 is a G3FAX machine, and at 30 is a telephone set. The ATM exchanger 27 by its nature is a device for transmission of digitized low speed to high speed multimedia signals such as data signals, voice signals, and image signals from data terminals or the PBX 28 out over a network, and description is made below for a case where a terminal (G3FAX machine 29, telephone set 30) is connected via the ATM exchanger 27 to the PBX 28.
The ATM exchanger 27 is connected to a public line network such as an ATM transmission path, a high speed digital private line or an ISDN line, and is used to improve efficiency in use of these lines due to the statistical multiplexed effect, and as a result, forms an ATM network.
In FIG. 9, a PBX 28 is connected to each of station A, station B, and station C as key points in this ATM network, and terminal equipment such as a G3FAX machine 29 and a telephone set 30 as network users are connected to the PBX 28 of each station. Each terminal can be connected via the PBX 28 to a terminal in another station by dialing the number of such terminal, but as a method of improving the line efficiency in the ATM network, in addition to the method based on the statistical multiplication effect, there is a method of further improving the efficiency in use of a line by identifying media transmitted from a terminal via the PBX 28 in the ATM network and executing voice compression processing for a voice (telephone set 30) or by converting the media to digital data and transmitting the data at a bearer speed.
FIG. 10 is a block diagram showing configuration of the ATM exchanger 27 as a general system based on the conventional technology, and in this figure designated at the reference numeral 1 is a device control section for controlling the entire ATM exchanger 27, at 2 a line interface section, and at 3 a PBX interface section.
Also designated at 4 is a device control section within the device control section 1 for monitoring trouble in a device and processing calls (talking path setting processing) or the like, and at 5 is an ATM bus control section for controlling the ATM bus 6.
And designated at 7 is a line interface section functioning as an interface with a line provided in the line interface section 2, at 8 a cell assembly/disassembly processing section for providing controls for STM/ATM exchange (STM=Synchronous Transfer Mode) or for ATM cells provided in the PBX interface section 3 as well as in the line interface section 2, at 9 an ATM bus exchange processing section for executing processing for switching to the ATM bus 6 (also provided in both sections 2 and 3), at 10 is a PBX line interface section provided in the PBX interface section for terminating an interface with the PBX 28.
Furthermore 11, 12 are selectors selectively connecting data to the voice processing section 13 or the FAX processing section 14 according to an instruction from the FAX protocol control section 15; 13 is a voice processing section for executing such processing as voice compression processing, at 14 is a FAX processing section for executing conversion of FAX data transmitted within the PCM sound band to digital data with a bearer speed, at 15 a FAX protocol control section which monitors data in the talking path and identifies media for each data and is capable of executing operations according to the G3FAX protocol based on the FAX protocol according to the ITU-T T.30. 16 is a tone detecting section for detecting a tone having a frequency of 2,100 Hz which is one of the requirements for G3FAX detection.
Next description is made of the operation. A connection request from terminal equipment such as the telephone set 30 or the G3FAX machine 29 is executed via the PBX 28 to the ATM exchanger 27. The ATM exchanger 27 constituting the ATM network terminates the connection request signal at the PBX line interface section 10 inside the PBX interface section 3 in a case where it functions as a relay exchanger according to the mode in use, and there are cases where a talking path is set by dialing the dial number and then data transmission is executed from the terminal, and where data is transmitted from the terminal through a talking path previously set irrespective of a particular connection request signal.
In either case, when a connection request is issued from a terminal and then data transmission is started, in the PBX interface section 3, the selectors 11, 12 select the voice processing section 13, where the signals are subjected to processing such as voice compression for sound data, and further processing in the cell assembly/disassembly processing section 8 and the ATM bus exchange processing section 9 required for ATM transmission are added thereto, and the signals are connected in a cell form to the ATM bus 6 for exchange.
As for the cell exchanged with the ATM bus control section 5 within the device control section 1 in the ATM bus 6, the contents of a cell control field required in the ATM bus exchange processing section 9 and cell assembly/disassembly processing section 8 are rewritten in the line interface section 2, and are sent via the line interface section 7 to a line. A cell received from a line is transmitted through a route reverse to that described above, namely via the line interface section 2, ATM bus 6, and PBX interface section 3 to the PBX 28 in a form of PCM sound data (STM), thus bidirectional voice communications are executed between terminals.
In this processing for communications as described above, generally the G3FAX machine 29 cannot transmit signals at an appropriate speed or becomes incapable of signal transmission due to voice compression processing or the like in the voice processing section 13. For this reason, in the PBX interface section 3, when communication is started, controls are provided so that the selectors 11, 12 select the voice processing section 13 assuming the communication media to be a sound signal as described above, but there also are provided the tone detecting section 16 and the FAX protocol control section 15 to identify other media during voice communication over a telephone line.
In a case where it is determined by tone detection (2,100 Hz tone or the like) or by 300 bps preamble detection with the control sections described above that the communication media is the G3FAX machine 29, the FAX protocol control section 15 provides controls for switching the selectors 11, 12 from the voice processing section 13 to the FAX processing section 14. By providing a FAX modem function in the FAX processing section 14, high efficiency transmission is realized in the ATM network as in the case of sound transmission at the bearer speed, by converting the sound band data of the G3FAX machine 29 to digital data.
The G3FAX transmission system in the conventional type of ATM exchanger is based on the configuration as described above, so that in the ATM network constructed with ATM exchangers as the base, transmission traffic becomes excessive, and in a case where a congested state is generated in the ATM network, signal transmission is executed by terminals operating at the G3FAX transmission rate, whereby cells are discarded in other traffic or in the G3FAX transmission itself, and for this reason transmission can not always be executed under stable conditions.